So I am helping a friend with a Harris HT30 issue, and suggested I'd ask the brain trust here for any input.

So this HT30 is randomly shutting down. Sometimes with no faults, other times with other random faults. No two times are a like (not the same fault indicator twice). After it shuts down, it attempts to restart a couple of times, then it will reset, and flame up like normal. You can apply the Plate voltage and it will be happy... for 45 minutes, and the random shut down will occur again. This is all with the TX in local. Doesn't matter if it's the primary or backup controller. The one note is the time, it is almost exactly 45 minutes between faults.

Harris mentioned it could be a flaky bad interlock switch, and they sent a package to replace them. Which he did, but the problem still exists.

So it's a crap shoot now and I mentioned I'd ask here.

(The Continental 816R-4 running as his backup, the 1991 vintage transmitter, is running smoothly!)

I had a similar issue with an HT20 several years ago. It had to do with the sensitivity of the overload circuit. There were some capacitor banks on the power line near the site, and whenever they'd do their thing the ripple on the line drove the controller of the HT20 nuts. For the life of me, I don't remember what the solution was (it was almost 15 years ago), but it resolved the issue.

So this HT30 is randomly shutting down. Sometimes with no faults, other times with other random faults. No two times are a like (not the same fault indicator twice). After it shuts down, it attempts to restart a couple of times, then it will reset, and flame up like normal. You can apply the Plate voltage and it will be happy... for 45 minutes, and the random shut down will occur again. This is all with the TX in local. Doesn't matter if it's the primary or backup controller. The one note is the time, it is almost exactly 45 minutes between faults.

First a couple questions:
1. When you say random faults, is there a frequently-found fault(s)? For example, do you always get a Plate Overload?
2. When the transmitter tries to restart, do you see the front panel meters do anything abnormal. Example: plate voltage rises, but no plate current? Plate current shoots up, then transmitter recycles? Play voltage meter shows nothing but transmitter recycles?

Here are what I've found will do something similar to your description:
A. Check the arc gap on the plate to ground. It's on the right side (looking in the rear of the PA cavity) of the aluminum portion of the plate capacitor. These things get burned over time and will carbon track. When the transmitter gets good and warmed up, it flashes over. Depending on how badly the acorn nuts are charred, you may be able to Scotch-Brite them clean. Reset the gap to about a nickle (on edge).

B. Depending how clean this box is kept, dirt will accumulate around the Plate blocking cap. This can cause arching through the Capton (brownish-orange color) sheet between the plate and ground. About the only way to check that is to remove the tube, unscrew the aluminum plate that is held down with Teflon 'L' shaped clamps, and while holding the orange Capton sheet up to a bright light, look for any holes. It could be the size of a pin. If you see a hole and you decide to replace the plate blocker, don't forget to clean up the aluminum where the hole was. There probably was carbon tracking. You don't want to replace the Capton, then blow another hole in it. If there is only one tiny hole, many times you can just tape a scrap piece of Capton over the hole and you're good to go.

C. You have a flakey HV power supply contactor. The coils will get weak over time and randomly drop, with very little ceremony. This is a Square D contactor, available through Square D electrical supply houses. Assuming GatesAir still has any, they will charge you a fortune. You may be able to just buy the coil assembly and rebuild the existing contactor.

Whatever you do, be careful when working around the high voltage areas on this transmitter. Over time the bleeder resistors open up and the HV supply may not bleed off when you turn the transmitter off. 'Chicken Stick' the $hit out of things.

First a couple questions:
1. When you say random faults, is there a frequently-found fault(s)? For example, do you always get a Plate Overload?
2. When the transmitter tries to restart, do you see the front panel meters do anything abnormal. Example: plate voltage rises, but no plate current? Plate current shoots up, then transmitter recycles? Play voltage meter shows nothing but transmitter recycles?

That's the funny part of the issue. No alarm is constant. Sometimes it's the plate overload. Sometimes it's other faults. Sometimes it's all. No single fault shows up every time. Metering is 100% normal on it, both the "analog" meters on top and the digital readings. Occasionally the "RF Mute" on the exciter will stick on as well.

Whatever you do, be careful when working around the high voltage areas on this transmitter. Over time the bleeder resistors open up and the HV supply may not bleed off when you turn the transmitter off. 'Chicken Stick' the $hit out of things.

Don't forget I work on a transmitter with much higher voltage as my normal "day job" and familiar with high voltage inside any tube transmitter. I appreciate the support and 'looking out for me' though. It is very kind of you.

1. Check the oil level in the "dashpot" breakers. For months, I chased a reoccurring fault on a 35 only to find that the installing engineer hadn't put the supplied dampening oil in the dash pots 10 years previous. Every little power line blip tripped the TX off at full power.

2. Replace the aux contacts on the PS contactors. That was a lot of years ago but I recall it being a very simple snap on/off aux contact that had been causing a weird intermittent problem for months. IIRC, Harris provided the replacement contacts as a kit.

As I recall, the dry dashpots caused the same fault every time but the aux contacts caused a variety of fault indications.

I've been scouring the manual for a particular thing and cannot find it but there are solenoids in the power supply that need periodic attention. I did find reference to how they work but nothing on the maintenance.

We had trouble with these on an HT35. Can't recall the symptoms whether they were similar to yours or not. Something to check at least.

Last time I had one of those boxes, there was an auxiliary contact on the HV or mains contactor that would lose full contact and open up to about 300 ohms. Just enough to shut it off. This was a HT or CD, can't remember which.

I'll second the comments about the auxiliary contacts on the high voltage contactor. The current through those contacts is very low, and if the status input it drives isn't satisfied then the transmitter drops off or overloads. I put a pull down resistor between the status input for those contacts and ground to force more current through the contacts, and that seemed to cure the problem.

Another problem that plagued me for a long time was a shutdown of the 700 watt IPA module. It turns out that the module will shut down with a B+ supply of 50 volts or greater, and there's a terminal on the barrier strip that will show that shutdown, but Harris ignored that status and didn't tie it to the controller. The fix was to retap the IPA power supply primary so that the voltage never exceeded 50 volts DC, with AC power line variations.

The Backup Controller has a solid state relay that was doing it. Apparently the relay on this controller is in service, even if the main is switched on.
My friend swapped out the relay and the problem has not returned.